Permanent magnetic structures, for example for the generation of a uniform magnetic field in a cavity, are known. Such structures are described, for example, in my publication "Optimum Design of Two-Dimensional Permanent Magnets", T.R. 21, NYU Medical Center, NYU School of Medicine, Oct. 15, 1989, and magnetic structures of this type that require yokes are described, for example, in my copending U.S. Pat. Application Ser. No. 07/591,458, filed Oct. 1, 1990.
In the design of such yoked magnets, it is sometimes inconvenient to provide means for accessing the cavity in the magnetic structure.